Examine the conformation of cyclopentane

•	This eclipsing effect explains why is there some angle strain in five membered rings even though the bond angles in the planar structure are almost 109.5 degrees.

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We now know that in the planar molecule there would be considerable strain caused by eclipsing of adjacent C-H bonds. Therefore the ring distorts (as in cyclobutane) to reduce these eclipsing interactions, but this increases the angle strain. Whatever happens there will always be some strain present. The minimum energy conformation adopted is therefore a balance of the two opposing types of strain.

•	There are two puckered conformations of cyclopentane, the envelope (IHIPOE, ACUHUB) and the half-chair (LISLOO, ABIKUR). There is little energy difference between the two forms and many five-membered ring systems adopt conformations somewhere between the two.

 

conformations of cyclopentane, left: envelope (IHIPOE); right: half-chair (LISLOO)